Porous photonic crystals for drug delivery to the eye

ABSTRACT

A minimally invasive controlled drug delivery system for delivering a particular drug or drugs to a particular location of the eye, the system including a porous film template having pores configured and dimensioned to at least partially receive at least one drug therein, and wherein the template is dimensioned to be delivered into or onto the eye.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/665,557, having a filing date of Feb. 17, 2009, which is a U.S.National Stage Application of International Application No.PCT/US2005/039177, filed Oct. 31, 2005, which application claimspriority to U.S. Provisional Application No. 60/623,409, filed Oct. 29,2004, the disclosures of which are incorporated herein by reference.

STATEMENT OF GOVERNMENT SPONSORED RESEARCH

This invention was made with Government support under grant no.F49620-02-1-0288 awarded by the Air Force Office of Scientific Research(AFOSR), under grant no. EY07366 awarded by the National Institutes ofHealth, and grant no. DMR-0503006 awarded by the National ScienceFoundation. The U.S. Government has certain rights in the invention.

FIELD OF THE INVENTION

A field of the invention is nanostructure synthesis. Other fields of theinvention include drug delivery, bioimplant materials and self-reportingbioresorbable materials.

BACKGROUND OF THE INVENTION

Diseases of the eye are numerous and frequently difficult to treateffectively. For example, some areas of the eye are difficult to reachwith systemic medications, while medications applied topically tend tobe transient and require numerous and repeated applications. Surgicaltreatment of still other diseases is invasive and often problematic aswell, with many patients ineligible for surgical treatment.

For example, intraocular diseases, such as age-related maculardegeneration (ARMD) and choroidal neovascularization (CNV), are theleading cause of irreversible vision loss in the United States, and yetcurrently available treatments for subfoveal CNV, which comprise themajority of CNV cases, are associated with only marginal visualimprovement and outcomes. As few as one quarter of patients with CNVassociated with ARMD are laser eligible, and at least half of thosetreated experience recurrence of the disease with poor visual outcomes.Similarly, photodynamic therapy using verteporfin is only useful for thesmall minority of patients with vessels that are angiographicallyclassified as “predominantly classic,” and even then the visual outcomesof such treatments are disappointing.

Pharmacologic therapy using local drug delivery or systemic drugdelivery is also being investigated using drugs that are antiangiogenic.Such drugs include angiostatic steroids, metalloproteinase inhibitorsand VEGF binding drugs. However, the problem common to all of thesepromising drugs is the transient nature of the therapeutic levelrequires frequent intravitreal injection.

Nonspecific uveitis is another devastating eye disease that affectsmillions of people in the world. Uveitis produces a wide spectrum ofinflammation of most parts of the eye and chronic uveitis can bedevastating in adults and children. Surgically implanted steroids haveshown that high intraocular doses for sustained times are extremelybeneficial to choronic uveitis patients, but this implant has surgicallyrelated side effects.

Intravitreal injection is being used in clinical trials of therapeuticagents, but pose a risk of infection that is estimated to be 0.5% perinjection. Due to the short vitreous half-life of most small moleculesafter intravitreal injection, frequent injection is needed, whichsignificantly increases the chance of intraocular infection.

Delivery of drugs into vitreous via liposomes or slow releasecrystalline lipid prodrugs extend the drug vitreous half-life, buttraditional liposomes or self-assembling liposomes often decreasevitreous clarity when used, can not be easily customized to releasedrugs with different physicochemical properties, and do not “report”drug release information.

Extraocular diseases are also difficult to treat because, for example,eye drops applied topically require repeated and frequent doses.

SUMMARY OF THE INVENTION

The invention provides minimally invasive controlled drug deliverysystems and methods for use in delivery of a particular drug or drugs tothe eye that include porous film or porous film particles having poresconfigured and dimensioned to at least partially receive at least onedrug therein. Embodiments include devices and methods for treatingintraocular diseases where porous film particles impregnated with aparticular drug are sized and configured to permit intraocular injectionof the loaded porous film particles. Other embodiments include devicesand methods for treating extraocular diseases, where one of a porousfilm, biodegradable polymer replica or porous Si-polymer compositeimpregnated with a particular drug is configured to contact a portion ofthe eye, such as the ocular surface or retrobulbar surface, andcontrollably release the drug for surface delivery of the drug.Advantageously, release of the drug is also monitorable such that theamount of drug remaining in the porous substrate can be accuratelyquantified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a chemical reaction for the oxidation of the porous Siaround a candidate molecule according to one embodiment of theinvention;

FIG. 2 illustrates a chemical modification reaction whereby a candidatemolecule is attached to an inner pore wall according to anotherembodiment of the invention;

FIG. 3 is a schematic diagram illustrating a templated synthesis ofpolymer photonic crystals using porous Si masters according to a firstembodiment of the invention; and

FIG. 4 is a graph illustrating a. correlation between the opticalthickness of an alkylated porous silicon film to the concentration ofdrug appearing in phosphate buffered-saline solution over 2 hours;

DETAILED DESCRIPTION OF THE INVENTION

The invention recognizes and addresses an important and unmet medicalneed for a minimally invasive, controllable and monitorable drugdelivery system and methods of using the system that would enable longacting local treatment of both extraocular and intraocular diseases.

For intraocular diseases, such as glaucoma, age-related maculardegeneration (ARMD), choroidal neovascularization (CNV), uveitis andothers, drug delivery to the vitreous, retina, and choroid is achallenging task due to the formidable obstacles posed by theblood-retinal barrier and the tight junctions of the retinal pigmentepithelium. Only small fractions of drug administered systemically reachthe target, requiring large and potentially toxic doses when deliveredsystemically. Another challenge to retinal drug delivery is the factthat drug levels should be sustained for prolonged periods at the targetsite. This is difficult using intravitreal injections because the shorthalf-life of most intravitreal injectable drugs. Intraocular implantshave provided sustained vitreoretinal drug levels for treating certainretinal diseases. However, this route demands intraocular surgery thatis known to cause intraocular complications when placing and replacingthe implant.

For extraocular diseases, such as viral keratitis, chronic allergicconjunctivitis, and scleritis, some of the same problems persist.Systemic administration of drug requires potentially toxic doses, andtopical treatments have a short half-life, requiring numerous andfrequent doses. Separately, photonic crystals have widespreadapplication in optoelectronics, chemical and biological sensors,high-throughput screening, and drug delivery applications. Thesephotonic crystals are especially advantageous because of the relativeease with which the optical properties, pore size, and surface chemistrycan be manipulated. Moreover, position, width, and intensity of spectralreflectivity peaks may be controlled by the current density waveform andsolution composition used in the electrochemical etch, thus renderingpossible the preparation of films of porous Si photonic crystals thatdisplay any color within the visible light band with high colorsaturation, which is a desirable feature for information displays.Traditional methods of intraocular drug delivery include the use ofliposomes or self-assembling liposomes, which often decrease vitreousclarity when used, cannot be easily customized to release drugs withdifferent physicochemical properties, and do not “report” drug releaseinformation.

Advantageously, the invention provides devices and methods for treatingboth intraocular and extraocular diseases that promote sustained releaseof a pharmacological candidate or drug, that is impregnated onnanostructured silicon, such as Si, SiO₂, Si/polymer or SiO₂/polymercomposite.

Preferred devices and methods are also self-reporting such that drugrelease and quantity remaining are susceptible of monitoring.Embodiments of the invention include minimally invasive, self-reporting,controlled delivery systems for delivering a drug or drugs to surfacesof the eyes, both the ocular surface (cornea and conjunctiva) and thescleral surface, as well as intraocular portions of the eye, includingthe retina, choroids, lens, ciliary body, anterior chamber, andvitreous.

A first preferred embodiment includes injection of porous microscopicnanostructured silicon particles impregnated with a particular drug ordrugs. While the invention contemplates use of numerous porousmicroscopic particles, preferred particles include porous silicon orsilicon dioxide particles (so called, “smart dust”), which are preparedwith a designed nanostructure that allows maintenance of sustainedintraocular therapeutic drug levels with minimal invasiveness andelimination of systemic side effects. In addition to configuring thenanostructure to suit individual applications, the invention alsocontemplates chemically modifying the particles and the particular drugor drugs to tune and control release profiles of the particles.Intraocular injection allows monitoring of drug levels non-invasively.

Porous silicon is especially advantageous in that porous silicon filmshave a large free volume (typically 50-80%), and thus a high capacityfor a drug can be custom designed at the nanoscale to deliver one ormore drugs at a variety of customizable release rates with multipledrugs, and the photonic properties of a nanostructured material as ameans to non-invasively determine the rate and amount of drug deliveredhas never been tested in the eye. The porous silicon photonic crystalparticles are impregnated with a particular drug, and subsequentlyintroduced into the retina, choroids, lens ciliary body, anteriorchamber, and vitreous of the eye via injection. For details of codedphotonic particles and methods of preparing same, see published U.S.application Ser. Nos.: 20050101026 entitled, “Photoluminescentpolymetalloles as chemical sensors,” 20050042764 entitled, “Opticallyencoded particles,” 20050009374 entitled, “Direct patterning of siliconby photoelectrochemical etching,” 20040244889, entitled, “Poroussilicon-based explosive,” and 20030146109 entitled, “Porous thin filmtime-varying reflectivity analysis of samples.” The “smart dust”photonic crystal particles may be optimized for intravitreal delivery ofone or more of a vast array of drugs such as, for example, pigmentepithelium derived factor (PEDF), an 8-mer peptide fragment of urokinase(uPA), dexamethasone, and a host of other drugs, small molecules,proteins, peptides and nucleic acids. These smart dust photonic crystalsmay be impregnated with drugs by either trapping one or more of thedrugs in porous Si smart dust, or second, the pores themselves may bechemically modified to bind the candidate drug.

Photonic crystals are produced from porous silicon and poroussilicon/polymer composites, or porous Si film or polymer replica orSi-polymer composite may be generated as a sheet for an exoplant. Pulsedelectrochemical etching of a silicon chip produces a multilayered porousnanostructure. A convenient feature of porous Si is that the averagepore size can be controlled over a wide range by appropriate choice ofcurrent, HF concentration, wafer resistivity, and electrodeconfiguration used in the electrochemical etch. This tunability of thepore dimensions, porosity, and surface area is especially advantageous.

The porous film is lifted off the silicon substrate, and it is thenbroken into micron-sized particles having a size conducive tointraocular injection. For example, in one preferred embodiment, themicron-sized particles are sized and configured such that they may beinjected into the eye with a 25 or 27-gauge needle. The particles act asone-dimensional photonic crystals, displaying an optical reflectivityspectrum that is determined by the waveform used in the electrochemicaletch. This spectrum acts as an optical barcode that can be observedthrough human tissue using, for example, an inexpensive CCD spectrometerand a white light source. For the drug delivery methods and systems ofthe invention, a drug is impregnated and trapped in the pores, and theoptical code may be used to report on the release rate of the drug inthe vitreous. In this manner, the amount of drug may be quantified todetermine how much remains within the particles, and whetheradministration of additional doses are necessary.

Advantageously, the optical interference spectrum used in particleidentification can be measured with inexpensive and portableinstrumentation (a CCD spectrometer or a diode laser interferometer).Removal of the drug from the pores is predicted to result in a change inthe refractive index of the porous film and will be observed as awavelength shift in the spectral code of the dust particle.Characteristic color changes are thus indicative of drug quantityremaining in the pores. Thus, the term photonic crystal is used for thefilm that has been machined and sized to small crystals for intraocularinjection.

For intraocular delivery of drugs, a doctor or clinician may lookthrough the iris of the eye and into the clear part of the eye toobserve the colors of the injected particles. In this manner, the amountof drug remaining or the degree to which the particles have dissolvedmay be monitored, which in turns permits the doctor or clinician toforecast the length of time before the particles completely dissolve,and to predict when the patient may need subsequent injections.

By way of example only, binding and release of a DNA 16-mer, IgG (usinga Protein A receptor) and biotinylated bovine serum albumin (using astreptavidin receptor) have been demonstrated using this methodology.The high surface area and optical interferometric means of detectionlead to very high sensitivity for many of these systems, and the factthat the materials are constructed from single crystal Si substratesmeans they can be readily prepared using Si microfabricationtechnologies.

In addition to having pore characteristics (thickness, pore size, andporosity) that may be controlled by the current density, duration of theetch cycle, and etchant solution composition, the porous silicon filmmay also be used as a template to generate an imprint of biologicallycompatible or bioresorbable materials. Both the porous silicon filmand/or its imprint possess a sinusoidally varying porosity gradient,providing sharp features in the optical reflectivity spectrum that havebeen used to monitor the presence or absence of chemicals trapped in thepores. It has been shown that the particles (smart dust) made from theporous silicon films by mechanical grinding or by ultrasonic fracturestill carry the optical reflectivity spectrum. These porous siliconparticles can be oxidized to increase stability and injected into animaleyes without toxicity to the intraocular tissues since silica is amineral needed by the body for building bones and connective tissue.Previous studies have demonstrated the biocompatibility of porous Si invitro and in animal models.

Other preferred embodiment include use of a porous silicon orsilicon/polymer composite at a particular location of the eye, or usingthe porous silicon or silicon/polymer composite as a template togenerate other biologically compatible or biologically resorbablematerials for similar use. Biodegradable polymer imprints may be madefrom porous silicon templates, which may be used as drug deliverycontact lenses or implants at an appropriate location of the eye,including the ocular surface and retrobulbar surface.

A second preferred embodiment of the invention include drug(s)impregnated in porous films configured to be worn or attached on thefront of the eye. A contact lens formed of impregnated porous thin filmmaterial, for example, comprises and embodiment of the invention. Whilethe second embodiment encompasses a contact lens, it also contemplatesother similarly curved solid template correspondingly shaped with afront surface of the eye, as well as being configured to join the eye atthe sclera as an episcleral plaque. The particular drug or drugs to beused with the polymer imprint may be added to the imprint solution priorto casting or engineered into the pores of the imprint after casting.

Accordingly, the second embodiment of the invention provides a systemand method of drug delivery wherein porous silicon films can bevariously modified to be a long-lasting intraocular drug deliveryvehicle to carry various therapeutic compounds. In addition,biodegradable porous polymer imprints made from porous silicon templatescan be used as a drug delivery implant to be placed at an appropriatelocation in the eye. The drug can be added into the imprint solutionbefore casting or engineered into the pores after casting.

For the extraocular drug delivery, the emphasis on optical reportingdeclines. With the episcleral plaque, for example, delivery isretrobulbar, and it is not as easy to use an optical instrument to“read” these films. In this retrobulbar embodiment, the ability of thenanostructure to set the rate of dissolution or drug release. Becausethe electrochemical process used to construct porous Si can control thenanostructure to such a precise degree, precise control of thedissolution and/or drug release profile of the particles or of thecomposites is conferred.

Thus, for example, the invention contemplates a contact lens configuredand arranged to cover a front extraocular surface, where a rim, or“carrier,” of the contact lens would be either a silicon orsilicon/polymer composite film impregnated with drug(s). The wearerwould receive a sustained and monitorable release of drug through thecontact lens.

Another preferred embodiment includes the use of episcleral plaques. Anepiscleral plaque is an extraocular way to deliver drugs and theintraocular dust Injection promotes monitoring of drug levelsnon-invasively. The invention contemplates use of a silicon orsilicon/polymer composite film impregnated with drugs to be affixed oradhered to a retrobulbar surface of the eye. The patient would therebyreceive a sustained and monitorable release of drug through theepiscleral plaque.

While the invention is contemplated for use with a virtually unlimitednumber of pharmaceutical candidates, several exemplary drugs will bediscussed herein.

For example, drug delivery for drugs used in treating ARMD and uveitiswill be shown for purposes of illustration. These diseases requireprolonged intraocular therapeutic drug levels to halt the progress ofthe disease and the deterioration of eyesight. However, the promisingdrugs for treating these diseases all share a common problem, which isthe transient intraocular therapeutic level requires frequentintravitreal injections. These promising drugs include angiostaticsteroids, metalloproteinase inhibitors, VEGF binding drugs, PEDF, an8-mer peptide fragment of urokinase (uPA) and dexamethasone. Inparticular, PEDF, the 8-mer peptide fragment of uPA and dexamethasoneall have short intravitreal half-lives.

Either silicon smart dust or the episcleral one-way releasing plaque ofbiodegradable polymer imprint of silicon smart dust provide a device andmethod for intravitreal drug delivery that promotes sustainedintraocular therapeutic drug levels with minimal invasiveness andelimination of systemic side effects.

Impregnation of the porous material may proceed in several ways. First,the candidate drug may be “physically” trapped within the pores, orsecond, the pores themselves may be chemically modified to bind thecandidate drug.

More specifically, “physical trapping” is similar to building a ship ina bottle, where the “ship” is the candidate drug and the “bottle” is thenanometer-scale pores in the porous Si matrix. Small molecules can betrapped in the porous matrix by oxidizing the porous Si around themolecule. The relevant reaction is illustrated in FIG. 1, where “O” inthe above equation is a molecular oxidant such as O₂, dimethylsulfoxide, hydrogen peroxide, or water. Since oxidation of silicon addstwo atoms of oxygen per atom of Si to the material, there is asignificant increase in volume of the matrix upon oxidation. This hasthe effect of swelling the pore walls and shrinking the free volumeinside the pores, and under the appropriate conditions, moleculespresent in the pores during oxidation become trapped in the oxidematrix.

The free volume in a porous Si film is typically between 50 and 80%.Oxidation should reduce this value somewhat, but the free volume isexpected to remain quite high. Most of the current drug deliverymaterials are dense solids and can only deliver a small percentage ofdrug by weight. The amount of drug that can be loaded into the porous Simaterial is expected to be much larger than, for example,surface-modified nanoparticles or polylactide (PLA) polymers.Experiments can quantify the amount of each of the drugs that can beloaded into the smart dust delivery vehicle.

During chemical modification, a molecule is attached to the inner porewalls via covalent bonds. The inner pore walls can be configured to bechemically modified by one of the group consisting of functionalalkenes, silicone oxide, functional organohalides, and metals. In theporous Si system, proteins, DNA, and various small molecules can beattached following several different procedures. The preferredembodiment uses electrochemical modification. For example, reduction of1-iodo-6-(trifluoroacetylamino) hexane at a p-type porous siliconcathode leads to attachment of the trifluoroacetamidohexyl group.Subsequent acid-catalyzed hydrolysis should lead directly to thesurface-bound amine species. The reactions are represented by theequation illustrated in FIG. 2.

The surface amine can then be functionalized with the 8-mer peptidefragment of uPA using standard peptide coupling methods.

The polymer replicas can be implanted on the sclera for trans-scleraldrug release. It has been shown in rabbit eyes that polymer replicas arebiocompatible and may safely and effectively remain in the eye formultiple months, if not years. Measurement of the decay in intensity ofthe peaks in the photonic crystal spectrum should provide a monitor ofthe rate of drug release from an implanted biocompatible polymer. Inorder to test the above hypothesis, drug-impregnated poly(L-lactide)(PL) films, cast from thermally oxidized porous silicon templates, canbe prepared following a scheme, designated generally at 10, illustratedin FIG. 3. Specifically, a template (such as electropolished PSi),generally at 12, is provided, having pores 14 dimensioned to suit aparticular application. A polymer, generally at 16, is loaded into thepores 14 to form a polymer-template composite. The template 12 issubsequently removed, leaving a polymer-based photonic film 16.

Replication of the optical spectrum in the biocompatible polymer uponremoval of the porous silicon template can be used to confirm thereplication process. The release characteristics of the polymers can bestudied.

The degradation of the photonic structure in these films can becharacterized in pH 7.4 aqueous buffer solutions, in vitro and in vivo.In accelerated degradation studies, we previously studied PL imprintsimpregnated with caffeine. We found that the intensity of the rugatepeak displays an approximately exponential decay when the polymer isdissolved in pH 10 buffer. Simultaneous measurement of the decay of thespectral peak and the appearance of caffeine in the solution (caffeineabsorption feature at 274 nm) confirmed that the drug was released on atime scale comparable to polymer degradation.

Embodiments of the invention also contemplate vectorial drug delivery.The polymer-based photonic film shown in FIG. 3 contains a polymer “cap”18 on one side of the film. Films prepared in this manner willpreferentially leach drug out one side of the film, allowing greatercontrol of the drug delivery parameters. Manufacturing variables arechannel sizes and packing.

Insofar as the invention contemplates including a virtually unlimitednumber of drugs, in vitro pharmacokinetic studies can be used todetermine the appropriate configuration of the porous silicon film andits dust for each drug. The drug conjugated porous silicon film and itsdust can be aliquoted into vitreous samples in cell culture dishes.Intensity of reflected light from the porous silicon film or its dustcan be measured using a low power spectrophotometer, at the same timefree drug in the vitreous sample can be measured, as a function of timefor the porous film or dust immersed in the vitreous sample. Correlationbetween spectrophotometer change and drug concentration in vitreous canbe determined and used for in vivo PK studies.

For biocompatible polymer imprints of the porous silicon film, drug canbe impregnated in the polymer casting solution. Then the free standingpolymer porous film can further conjugate with drug molecules to fillthe pores. In vitro PK studies can be performed in a similar way as withthe porous silicon film or its dust.

Optimized porous silicon smart dust adapted to the drug candidate willnot be toxic after intravitreal injection and the vitreous drughalf-life will be in the range of weeks and the drug level will sustainabove the EC for months.

A preferred method includes preparing porous Si photonic crystalparticles, loading the pores of those crystal particles with one or moredrugs, and injecting the particles into the vitreous via syringe. Theamount of drug loaded in the particles may then be monitored via one ormore of a plurality of methods, such as by visual inspection, digitalimaging, laser eye scan, or spectroscopic observation. Any of these fourmethods are non-invasive, allowing the practitioner or clinician toobserve the particles through the pupil of the eye.

More particularly, one preferred method of the invention proceeds asfollows. Porous Si photonic crystals are formed from a porous siliconfilm that is electrochemically etched in a single crystal Si substrateby application of a sinusoidal current density-time waveform. Thewaveform varies between 15 and 45 mA/cm², with 70 repeats and aperiodicity of 12.5 s. The one-dimensional photonic crystal that resultshas a color that depends on the waveform parameters. The conditionsdescribed above produce a film that has a strong reflectivity maximum inthe green region of the spectrum. This is a convenient color for visualobservation in the eye, though any color or pattern of colors (multiplespectral peaks) can be incorporated into the films. The spectralfeatures can range in wavelength from 300 nm to 10,000 nm. The film isremoved from the Si substrate using a pulse of current. Particles withdimensions in the range 1 μm to 270 μm are generated by ultrasonication.

The photonic crystals are then loaded with a drug or drugs. The pores ofthe photonic crystals are large enough to allow infiltration of smalldrugs such as dexamethasone. Drug can be loaded into the film orparticles by infiltration from solution. In a typical preparation, thedrug loading solution consisted of 6.times.10-2 M dexamethasone inmethanol. 25 μL of the solution was pipetted onto the porous Si film andthe solvent was allowed to evaporate in air. The film was briefly rinsedwith deionized water to remove any excess drug remaining on the surfacethat had not infiltrated the pores.

Once the drug is loaded into the pores of the photonic crystals, thephotonic crystals are then injected into the patient. The drug-loadedcrystals are placed in an appropriate excipient and injected into thevitreous. After intravitreal injection, the porous silicon particlesfloated in the vitreous affording an ophthalmoscopically clear view ofthe fundus without any observed toxicity. The particles may last in thevitreous for up to four months without any noticeable abnormalities.

The optical interference spectrum used in particle identification canreadily be measured with inexpensive and portable instrumentation suchas a CCD spectrometer or a diode laser interferometer. Removal of thedrug from the porous nanostructure results in a change in. therefractive index of the porous film and is observed as a wavelengthshift in the spectrum, or a shift in the code, of the dust particle. Thehigh surface area and optical interferometric means of detection lead tovery high sensitivity for this system. Furthermore, particles can beencoded to reflect infrared light that can penetrate living tissues andenable noninvasive sensing through opaque tissue.

Experimental Data and Results:

Porous silicon dust was injected into rabbit vitreous and no toxicitywas found compared with the fellow eyes that received the same volume ofphosphate-buffered saline (PBS) injection. The porous silicon film wasetched using a sinusoidal current varying between 15 and 45 mA/cm², with70 repeats and a periodicity of 12.5 s. The film was sonicated into adust that ranged from 1 μm to 270 μm. After intravitreal injection, theporous silicon particles floated in the vitreous affording anophthalmoscopically clear view of the fundus without any observedtoxicity. The particles lasted in the vitreous for one week without anynoticeable abnormalities.

Thermally oxidized silicon dust was also injected into the vitreous offour rabbits. This chemical modification of the porous silicon film wasproposed as one of the alternative methods to increase the residencetime of the porous silicon dust in vitreous. This approach demonstrateda great increase of the residence time of the particles in the rabbiteye compared to the previous incompletely hydrosilylated smart dust(from less than 7 days to longer than 3 weeks). In addition, byincreasing the sonication time during preparation, smaller and moreuniform smart dust particles were produced, which can be delivered intovitreous by the 25 or 27-gauge needle that is commonly used forintravitreal injection in the clinic.

Additional data supports use of completely hydrosilated porous Siphotonic crystals that have no toxicity by clinical examination orelectroretinograms or histology at 3½ months post injection, inclusiveof shorter times. For example, 100 microliters of the material wereinjected, and the characteristic color of the crystals is seen making itclear that one can use this characteristic for monitoring drug releasein the eye.

Intravitreal injection of 100 μl of oxidized porous Si photonic crystalparticles in 5% dextrose was performed. The measured size of the smartdust ranged from 10 to 45 μm with an average of 30 μm; approximately30,000 particles were injected into each rabbit eye. The injectedparticles appeared purplish green floating in the vitreous. From thesecond day some of the particles aggregated and sank onto the inferiorretina. No toxicity was seen and the smart dust particles were stillvisible at the last examination 34 weeks later with at least half of theoriginally injected material remaining, as assessed by ophthalmoscopy.It is therefore anticipated that the particles would be safe andeffective for at least a year if not two years. Thus, this preliminarythermal oxidation modification has greatly extended the time ofintravitreal residence compared to the previous incompletelyhydrosilylated smart dust.

The data demonstrated that the porous silicon particle was safe as anintravitreal drug delivery vehicle. Modifications such as oxidation andsilicon-carbon chain conjugation can be used to further increase thestability of the silicon dust and can make it a long-lasting slowrelease intravitreal drug delivery system.

A preliminary study was performed on a rat CNV model using systemicadministration of an 8-mer peptide derived from urokinase plasminogenactivator (uPA) to block the uPA-urokinase plasminogen activatorreceptor (uPAR) interaction. This 8-mer peptide was administratedsubcutaneously twice daily at 200 mg/kg/d beginning at the time ofinduction of CNV (with laser) to introduce CNV in Brown Norway rats. Twoweeks after laser treatment, simultaneous FA and ICG using scanninglaser angiography was performed to identify the leaking laser burns. Theresults showed that this 8-mer peptide reduced the laser induced CNV by70% compared to the control group (44.7% of laser burns leak in controlgroup versus 13.4% in treated group, p<0.001). [55] Administration ofthe drug intravitreally using a proposed porous silicon smart dustshould maintain the desired intraocular drug level.

Thermal Oxidation of Porous Si Particles

Preliminary studies of porous Si particles oxidized and annealed at 300°C. for 2 hours in air show that the material is stable in aqueous pH 11buffer for several days, and recent results indicate that this approachcan dramatically increase the residence time of the particles in therabbit eye. In addition, by increasing the sonication time duringpreparation, smaller and more uniform smart dust particles were producedwhich can be delivered into vitreous by the 28.5 gauge needle that iscommonly used for intravitreal injection in the clinic. Intravitrealinjection of 100 μl of oxidized porous Si photonic crystal particles in5% dextrose was performed. The measured size of the smart dust rangedfrom 10 to 45 μm with a average of 30 μm; approximately 30,000 particleswere injected into each rabbit eye. The color of the injected particlesfloating in the vitreous was clearly visible, which is indicative ofdrug release and degradation by hydrolysis. Degradation by hydrolysis isespecially advantageous in that no enzymes are necessary to degrade theparticles. From the second day some of the particles aggregated and sankonto the inferior retina. No toxicity was noticed and the smart dustparticles were still visible until the last examination, which indicatesthat this preliminary thermal oxidation has more than tripled the timeof intravitreal residence compared to the previous incompletelyhydrosilylated smart dust. Experiments can be performed to quantify theresidence time and correlate it with tile chemical modificationconditions such as thermal oxidation time, temperature, and ambientatmosphere.

Electrochemical Grafting of Organic Reagents

The hydride-terminated surface of p-type or p⁺⁺-type porous silicon canbe stabilized by electrochemical reduction of acetonitrile solutions ofvarious organo halides. Reduction of 6-iodo-ethylhexanoate,1-iodo-6-(trifluoroacetylamino) hexane, iodomethane, 1-bromohexane, orethyl 4-bromobutyrate at a porous Si cathode results in removal of thehalogen and attachment of the organic fragment to the porous Si surfacevia a Si—C bond. A two-step procedure was devised involving attachmentof the functional group of interest followed by attachment of methylgroups (by reduction of iodomethane) to residual, more stericallyinaccessible sites on the porous Si surface and found thatelectrochemical alkylation greatly improves the stability of porous Siagainst oxidation and corrosion in various corrosive aqueous media, andthat the methyl capping procedure provides the most stable porous Simaterial yet reported. This chemistry also allows covalent attachment ofthe candidate drugs for the release studies.

Thermal Hydrosilylation of Organoalkenes

This approach provides a porous Si material that is stable even inboiling aqueous pH 10 solutions. This chemistry was extended to the dustparticles and find similar levels of stability. Parameters of thereaction may be adjusted in order to identify the key parameters leadingto this instability. In particular, the surface coverage (essentiallythe efficiency of the chemical reaction), the type of organic speciesgrafted to the surface (alkyl carboxylates, alkyl esters, and alkylhalides), and the chain length of the alkyl species can be investigated.Reaction conditions such as the presence of added radical initiators,transition metal catalysts, and photoassisted hydrosilylation can beexplored.

For each modified porous silicon film, its sonicated dust can beintravitreally injected into 3 rabbit eyes with the fellow eyes used forcontrol. After injection, the toxicity can be monitored by slit lamp,indirect ophthalmoscope, ERG, and pathology. In addition, a remotespectrometer probe can be used to determine the clearance rate of thesilica dust in vitreous on living animals through the dilated pupil. Thespectrometer probe is believed to render more accurate information sincethe small particles may not be seen using indirect ophthalmoscope.

A spectrometric method of detection of the oxidized “smart dust”injected into the rabbit eyes was also investigated. One eyepiece of thesurgical microscope was connected to the input of a fiber-optic basedspectrophotometer and this allows us to accurately focus the detectinglight on the intraocular “smart dust” particles. The preliminary datashowed a feasibility of this approach and the specific wavelength of aporous Si photonic film was detected with a 1 nm spectral resolution.This resolution is sufficient to determine concentration of a speciessuch as a large protein in the porous Si film to micromolarconcentration levels. As an alternative, the probe can be adapted to afundus camera which is used for clinical retinal imaging. For the rabbitor rodent eyes, the fundus can be photographed using a fundus camerawithout anesthesia.

In in vitro experiments, the optical codes of the porous Si photoniccrystal particles can be read using digital imaging cameras. Since thecolor of the particles provides an indirect measure of the amount ofdrug loaded, the most accurate measure is obtained using a spectrometer.However, the color resolution in a digital camera is sufficient tomeasure the loading to an accuracy of 10%, which is sufficient for thepresent application. In order to measure the degree of loading in porousSi “smart dust,” the color of the particles can be recorded using acolor digital camera connected to the fundus camera. Software to processthe digital images and extract concentration information can be obtainedwith minor modifications to commercially available software. Theadvantage of this approach is that it requires only minor modificationto existing readily available medical equipment, and it allowsacquisition of data from a large number of particles simultaneously. Ifhigher resolution concentration information is needed, the illuminationlight can be filtered using a monochromator or bandpass filters,providing spectral resolution equivalent to that which can be obtainedwith a spectrometer.

The long-lasting porous silicon film and its imprint can be furtheroptimized for delivery of three candidate drugs (PEDF, an 8-mer peptidefragment of uPA, and dexamethsasone) by controlling the pore size andmorphology. These parameters are easily controlled using the appropriateanodic electrochemical etching current density, duration of the etchcycle, and etchant solution composition. Since the imprint and itsporous silicon template share the similar nanostructures, it is assumedthat imprints from optimized porous silicon can also be appropriate fordelivering those drug candidates.

Additional in vivo data regarding the “smart dust” material afterintraocular injection and new in vitro data concerning the release ofdexamethasone from “smart dust” formulations is as follows.

In Vivo Studies

The new formulation of “smart dust” particles containing a silicondioxide shell have been observed in the vitreous of living rabbits for16 weeks and they are showing evidence of dissolution without anyevidence of toxicity by slit lamp, indirect ophthalmoscopic examinationsor by light or electron microscopy. More than half of the particlesappear to be present at this time point indicating excellent potentialas a long acting drug delivery system. Injection of “smart dust”particles containing a hydrosilylated alkyl shell into the living rabbiteye has shown no evidence of toxicity for up to five weeks of ongoingexamination.

Additional in vivo studies demonstrated the increased stability of“smart dust” particles containing a hydrosilylated alkyl shell. Thesechemically modified particles also exhibit slower release rates for adrug. Release of dexamethasone from the modified porous silicon matrixis slowed by a factor of 20 compared to unmodified porous silicon.

Chemistries have also been developed to expand the pores in order toaccommodate larger molecules within the pores, such as a modified Fabfragment of human IgG. The pore expansion procedure involves theenlargement of pores by treatment with dimethylsulfoxide (DMSO)containing hydrofluoric acid (HF). The porosity increases approximately10% after the expansion treatment, and it was found that this chemistryallows admission of large molecules such as human IgG (150 kDa) andbovine serum albumin (67 kDa).

As will be clear to artisans, the invention makes use the opticalproperties of porous silicon photonic crystals to monitor drug deliveryrates. The shift in the reflectivity spectrum of the film coincides withrelease of a drug. Optical measurements were carried out whileconcurrent absorbance measurements were obtained as the drug-infusedporous silicon films were introduced in buffered aqueous solutions.There is a linear correlation between the increase of drug concentrationin solution (i.e. drug diffusing from the pores) and a change in theoptical thickness of the porous silicon film.

While various embodiments of the present invention have been shown anddescribed, it should be understood that modifications, substitutions,and alternatives are apparent to one of ordinary skill in the art. Suchmodifications, substitutions, and alternatives can be made withoutdeparting from the spirit and scope of the invention, which should bedetermined from the appended claims.

Various features of the invention are set forth in the appended claims.

What is claimed is:
 1. A drug delivery device for use in the controlleddelivery of a particular drug or drugs to a particular location of theeye, the device comprising: micron sized porous silicon or silicondioxide particles having pores configured and dimensioned to at leastpartially receive at least one drug therein; and wherein the particlesare suitable to be delivered into or onto the eye.
 2. The device ofclaim 1, wherein the inner walls of the pores are covalently modified sothat the binding efficacy of the at least one drug is enhanced and/ordrug release profiles of said pores has been tuned.
 3. The device ofclaim 2 wherein the covalent modification of the inner walls is selectedfrom the group comprising functional alkenes, silicon oxide, functionalorganohalides, and metals.
 4. The device of claim 1 wherein theparticles are oxidized so as to trap the drug or drugs in the pores. 5.The device of claim 1 wherein the particles are suitable for intraocularinjection.
 6. The device of claim 1, wherein said particles have amonitorable optical code.
 7. The device of claim 1 wherein said drug ordrugs comprises one of the group consisting of angiostatic steroids,metalloproteinase inhibitors, a VEGF binding drug, pigment epitheliumderived factor, an 8-mer peptide fragment of urokinase, anddexamethasone.
 8. A method of preparing a device for controlled drugdelivery to a location of the eye comprising: providing a porousnanostructured silicon-containing template having pores configured toreceive a particular drug, fracturing the template into micron sizedparticles, said particles being sized and configured to be deliveredinto or upon a surface of the eye; and loading either the template orthe micron sized particles with the drug.
 9. The method of claim 8wherein the particles are suitable for injecting intraocularly.
 10. Themethod of claim 9 wherein the particles have a monitorable opticalresponse depending on the quantity of drug disposed in the pores. 11.The method of claim 9 wherein the particles have a monitorable opticalresponse depending on the amount of porous material present.
 12. Themethod of claim 8 further comprising trapping the drug or drugs in thepores by oxidizing the porous template around the drug or drugs.
 13. Themethod of claim 8 further comprising covalently modifying the innerwalls of the pores to enhance binding efficacy of the at least one drugand to tune release profiles of said pores.
 14. A micron sized poroussilicon or silicon dioxide particle having pores configured anddimensioned to at least partially receive at least one drug therein; andwherein the particle is suitable to be delivered into or onto the eye.15. The particle of claim 14, wherein the inner walls of the pores arecovalently modified so that the binding efficacy of the at least onedrug is enhanced and/or drug release profiles of said pores has beentuned.
 16. The particle of claim 14, wherein said drug is selected fromthe group consisting of angiostatic steroids, metalloproteinaseinhibitors, a VEGF binding drug, pigment epithelium derived factor, an8-mer peptide fragment of urokinase, and dexamethasone.